Proanthocyanidin (grape seed extract) has been studied for its usefulness as a therapeutic agent for blood vessels, and one of the reasons for recent progress in these studies is that the target substance can serve as a marker for tracing in vivo absorption and metabolism because it is stable against heat and acids, highly soluble in water and highly absorbable in the body. In contrast, polyphenol compounds such as catechin are often difficult to dissolve in water, and also involve a problem in that they are less absorbable in the body.
Attempts have been made to develop a technique for glycosylation of catechin and other compounds, with the aim of improving their solubility in water and increasing their stability.
By way of example, Patent Document 1 discloses α-glucosidase with a molecular weight of about 57,000, which was collected from a culture solution of Xanthomonas campestris WU-9701. This enzyme uses maltose or the like as a donor (does not use maltotriose, cyclodextrin or starch as a donor) and transfers glucose to a specific acceptor to synthesize a glycoside. In this document, compounds listed as acceptors are those having an alcoholic hydroxyl group (e.g., menthol, ethanol, 1-propanol, 1-butanol, 2-butanol, isobutyl alcohol, 1-amyl alcohol, 2-amyl alcohol, 5-nonyl alcohol), as well as those having a phenolic hydroxyl group (e.g., capsaicin, dihydrocapsaicin, nonylic acid vanillylamide, catechin, epicatechin, vanillin, hydroquinone, catechol, resorcinol, 3,4-dimethoxyphenol). Moreover, glycosides whose production was actually confirmed are monoglucosides only.
Patent Document 2 discloses a method in which a mixture of a catechin compound and glucose-1-phosphate or sucrose is treated with sucrose phosphorylase to prepare a glycoside of the catechin compound. The sources of sucrose phosphorylase listed therein are Leuconostoc mesenteroides, Pseudomonas saccharophila, Pseudomonas putrefaciens, Clostridium pasteurianum, Acetobacter xylinum, and Pullularia pullulans. Likewise, catechin compounds listed as acceptors are (+)-catechin, (−)-epicatechin, (−)-epicatechin 3-O-gallate, (−)-epigallocatechin and (−)-epigallocatechin 3-O-gallate, but it is only (+)-catechin that was actually used as an acceptor to prepare (+)-catechin 3′-O-α-D-glucopyranoside in the Example section.
Patent Document 3 discloses epigallocatechin 3-O-gallate derivatives, in which a glucose residue or a maltooligosaccharide residue with a polymerization degree of 2 to 8 is attached to at least one of the 5-, 7-, 3′-, 4′-, 5′-, 3″-, 4″- and 5″-positions. As in the case of Patent Document 2, the Example section of Patent Document 3 actually discloses only a case where a mixture of (−)-epigallocatechin gallate and glucose-1-phosphate or sucrose was treated with sucrose phosphorylase to prepare 4′-O-α-D-glucopyranosyl(−)-epigallocatechin gallate and 4′,4″-O-α-D-di-glucopyranosyl(−)-epigallocatechin gallate.
Patent Document 4 discloses tea extracts or tea beverages whose astringent taste is reduced by glycosylation of polyphenols contained therein. To reduce the astringent taste of tea extracts or tea beverages, this document describes detailed procedures in which tea extracts or tea beverages are supplemented with dextrin, cyclodextrin, starch or a mixture thereof, and then treated with cyclomaltodextrin glucanotransferase. In the Example section, it is shown that a green tea extract and α-cyclodextrin were treated with cyclomaltodextrin glucanotransferase derived from Bacillus stearothermophilus to give a reaction product with reduced astringent taste, which in turn indicates that polyphenols such as epigallocatechin 3-O-gallate and epicatechin were glycosylated. However, this document fails to show the detailed structure of the reaction product.
Patent Document 5 discloses glycosides of catechin compounds in which glycosylation occurs at the 3′-position, at the 3′- and 5-positions, or at the 3′- and 7-positions. For this purpose, this document describes detailed procedures in which a catechin compound and dextrin, cyclodextrin, starch or a mixture thereof are treated with cyclomaltodextrin glucanotransferase derived from Bacillus stearothermophilus, as in the case of Patent Document 4. Further, in the examples using dextrin as a glycosyl donor in the above procedures, some of the resulting glycosides of (−)-epigallocatechin, (−)-epigallocatechin 3-O-gallate and (−)-epicatechin 3-O-gallate are considered to have 6 to 8 glucose residues on average per molecule of each polyphenol, as determined from their molar absorption coefficients. Moreover, it is confirmed that upon treatment with glucoamylase derived from Rhizopus niveus, the glycosides obtained by the above procedures generated 3′,7-di-O-α-D-glucopyranosyl(−)-epigallocatechin, 3′,5-di-O-α-D-glucopyranosyl(−)-epigallocatechin, 3′-O-α-D-glucopyranosyl(−)-epigallocatechin, 3′,7-di-O-α-D-glucopyranosyl(−)-epigallocatechin 3-O-gallate, 3′-O-α-D-glucopyranosyl(−)-epigallocatechin 3-O-gallate, 3′-O-α-D-glucopyranosyl(−)-gallocatechin and 3′-O-α-D-glucopyranosyl(−)-epicatechin 3-O-gallate.
As to effects provided by catechin glycosides, Non-patent Document 1 describes reduced astringent taste, increased water-solubility, improved stability and inhibited tyrosinase, while Non-patent Document 2 describes suppressed mutagenicity.                Patent Document 1: JP 2001-46096 A        Patent Document 2: JP 05-176786 A (Japanese Patent No. 3024848)        Patent Document 3: JP 07-10897 A (Japanese Patent No. 3071610)        Patent Document 5: JP 09-3089 A (Japanese Patent No. 3712285)        Non-patent Document 1: Biosci. Biotech. Biochem., 57 (10), 1666-1669 (1993)        Non-patent Document 2: Biosci. Biotech. Biochem., 57 (10), 1290-1293 (1993)        